The first step that I did on building a Roomba-based robot was removing the vacuum. This will reduce weight, and give space for a big caster in the back.
Step 1: Get a Used Roomba
You can find used Roomba's at thrift stores, classified ads, etc. If you look for ones where the "vacuum doesn't work", you can save yourself some money because hey, you don't need a vacuum. Don't pay over $50 bucks for one though if you are going to use it for a robot. Just get a Create instead. But I've gotten about 5 of them for $40 or under over the years.
Step 2: Remove Battery, Dust Bin, and Brushes.
Flip the Roomba over. Remove the battery, the dust bin, brush holder, and brushes.
Step 3: Remove the Bumper
There are 4 philips-head screws that need to removed from the bottom to allow the bumper to be removed. Remove these and then flip the Roomba over. Carefully pull the bumper away from the Roomba until you feel some resistance from the cable. Unplug the cable from its connector.
Step 4: Remove the Top Cover
Flip the Roomba back over and remove the marked screws to remove the top cover. Pretty much if you can see a screw, then take it out. Flip the Roomba over again and carefully remove the top panel until you feel resistance from the cover. Unplug the top panel plug from its connector.
Step 5: Remove the Vacuum
Now look at the Roomba top down. Unscrew the screw holding the rope that holds the vacuum in (first pic). Then unhook the connectors that connect the vacuum motor and well, I don't know what that thing is, but we don't need it. Now, unscrew and pull out the two plastic things at the bottom end of the bin. You should be able to pull out the vacuum assembly.
Step 6: Put It Back Together
Now that the vacuum is removed, time to put it back together. Put the plastic tabs in the back back on. Reconnect the top panel connector and screw back in the top panel. Do the same for the bumper.
Step 7: Tear Down the Dust Bin
I tore down my dust bin to make it easier to put on a caster wheel. Unscrew the bottom with your philips screwdriver. Take it apart and you will see four more screws, unscrew these. Take the top and connect it with your bot. Glue, tape, wood, whichever you please.
Step 8: Final Product
Put the battery back in and start hacking!